A Sydney man who inflicted "wanton violence" on his two-year-old stepson and let him suffer extreme pain for hours before killing him with a blow to the head has been sentenced to at least 20 years and six months in jail.

Gregory Wayne Hill, 40, murdered Jakob Brown on March 3, 2008, at The Oaks in Sydney’s south-west. He pleaded guilty to murder on November 11 last year, a week before his trial was to commence.

In the NSW Supreme Court on Thursday, Hill received less than the standard non-parole period of 25 years' jail after Justice Geoffrey Bellew took into account a number of mitigating factors including his guilty plea and his ongoing psychological problems and severe back pain.

Justice Bellew said Hill, who was in a de facto relationship with Jakob’s mother, had seriously assaulted the little boy on at least four previous occasions and then lied to cover up the abuse.

Jakob’s injuries and symptoms were explained as the result of falls or a pre-existing medical condition.

On the day of his death, Jakob’s mother left him in Hill’s care while she went to a TAFE course.

Hill inflicted a "ferocious blow" to his abdomen that caused a gastric tear, which caused the contents of his stomach to spill into his abdomen and would have caused Jakob severe pain.

Justice Bellew said Jakob would have been "demonstrably unwell" but, rather than seek the medical help he so obviously needed, Hill left the "defenceless and helpless" toddler suffering in pain for hours before striking a blow to his head causing a massive skull fracture.

"Rather than get medical treatment, he chose to inflict further injuries which ultimately led to Jakob’s death," Justice Bellew said.

Justice Bellew said there were ‘‘two separate and distinct acts of wanton violence were perpetrated by the offender upon a helpless and defenceless infant".

"Following the first, Jakob was left in pain. Following the second, he was left to die. In acting as he did, the offender showed a callous disregard for Jakob’s life."

Jakob was eventually found by another family member covered in blood and vomit in his cot. He was taken to Campbelltown Hospital but could not be revived.

During a sentencing hearing, the court heard Jakob could have survived the gastric tear had he received prompt medical care.

"For reasons which remain completely unexplained, the offender’s response was not to ensure that care was administered," Justice Bellew said.

Hill’s claim that Jakob had two minor falls but was "fine" before his death was comprehensively disproved by the forensic evidence that showed the injuries could only have been the result of the infliction of grievous bodily harm.

Hill suffers from chronic pain from a back injury sustained in 2000 and a car crash in 2005 and also has post-traumatic stress disorder, anxiety and depression.

During sentencing, his lawyers told the court his pain and psychological conditions were not being adequately addressed and Justice Bellew ordered Justice Health look into his treatment.

Hill was sentenced to a maximum of 27 years and six months in jail.

Outside the court, Jakob’s father Darren Gibson said: "Greg Hill, you’re a piece of shit, that’s all you are.

"You don’t deserve to go to jail. You deserve a death penalty and that’s it.

"I hope he rots in hell for the rest of his life."

Jakob’s mother Tracey Brown, said Jakob was "gorgeous".

"He was beautiful. And he didn’t deserve what happened to him."

Detective Sergeant David Schofield said the family had been reported to child welfare authorities and a number of notifications were made to the then-Department of Community Services (DOCS).

He said the response of authorities to the "pattern of abuse" and "why things weren't done earlier" will be considered at an upcoming inquest into Jakob's death.

He said the police investigation was "lengthy, difficult and confronting".

"I can only imagine what that poor child went through in those hours leading up to his death," he said.